Numbers, breeding chronology, and breeding success of Common Murres at Bluff, Alaska, in 1975–1991

Abstract

In 1979–1991, midsummer counts of Common Murres (Uria aalge) at Bluff, Alaska, showed no overall trend but varied in relation to weather conditions on the days of the counts within years and also varied markedly among years in relation to spring weather conditions, timing of breeding, and the number of actively breeding pairs. The high variability in annual mean counts during a period when there was no overall change in numbers indicates that annual differences in counts may reflect differences in colony attendance of adults rather than changes in population size. Both breeding chronology and reproductive success varied among years: breeding chronology was most advanced and reproductive success was highest in years when spring air temperatures were highest. Annual variations in numbers, breeding chronology, and breeding success paralleled those for Black-legged Kittiwakes, but the magnitude of variability in murre numbers and breeding success was much lower than that of kittiwakes. Breeding chronology of Common Murres was earlier at Bluff than at coastal colonies north of Bluff and island colonies in the central Bering Sea; however, comparisons with other studies indicate that there are no easily discerned geographic patterns. Although Bluff is near the northern limit of the breeding range of Common Murres in the eastern Pacific Ocean, there is no evidence of adaptations to shorten the duration of breeding attempts or of a shortened breeding season. In contrast to studies of Common Murres elsewhere, we found that reproductive success did not decline seasonally and breeding success of replacement attempts equaled that of first attempts. Both the very earliest and latest attempts were unsuccessful each year, however, indicating stabilizing selection on the laying dates of murres at Bluff.